docs: run how-to guides in CI (#27615)

Add how-to guides to [Run notebooks
job](https://github.com/langchain-ai/langchain/actions/workflows/run_notebooks.yml)
and fix existing notebooks.

- As with tutorials, cassettes must be updated when HTTP calls in guides
change (by running existing
[script](https://github.com/langchain-ai/langchain/blob/master/docs/scripts/update_cassettes.sh)).
- Cassettes now total ~62mb over 474 files.
- `docs/scripts/prepare_notebooks_for_ci.py` lists a number of notebooks
that do not run (e.g., due to requiring additional infra, slowness,
requiring `input()`, etc.).

docs/scripts/cache_data.py

@@ -0,0 +1,14 @@
+import tiktoken
+from unstructured.nlp.tokenize import download_nltk_packages
+
+
+def download_tiktoken_data():
+    # This will trigger the download and caching of the necessary files
+    _ = tiktoken.encoding_for_model("gpt2")
+    _ = tiktoken.encoding_for_model("gpt-3.5-turbo")
+    _ = tiktoken.encoding_for_model("gpt-4o-mini")
+
+
+if __name__ == "__main__":
+    download_tiktoken_data()
+    download_nltk_packages()